The piston effectively forms a pressure tight plunger sliding up and down the cylinder and the piston features a piston skirt, whereas the outer surface of the skirt slides along the cylinder running surface in a longitudinal axis corresponding to the longitudinal axis of the piston. In its center area the piston features an inner section and a piston surface on top of the piston body, which limits the combustion chamber and forms a continuous transition area to the upper area of the piston skirt, whereas a piston crown bowl is arranged in the topography of the top surface and is a part of the transition area. In the lower end the inner section crosses over into the joint claw which is part of the connecting rod assembly and the connecting rod is linked to the piston by means of negative locking dispensing a piston pin.
The piston converts the pressure provided by the combustion process into a reciprocating mechanical movement. By connecting the reciprocating piston to a crankshaft with a connecting rod, the reciprocating movement is converted into rotational movement of the crankshaft. The connecting rod connects the piston to the crankshaft, and the connecting rod is therefore subject to high forces exerted on the piston by the combustion pressure. When the combustion pressure forces the piston down the cylinder, the connecting rod must transfer the force through to the rotating crankshaft.
Some regions or parts of the piston are therefore subject to different mechanical forces and loads leading to corresponding mechanical expansion and/or retraction in the piston skirt. The piston skirt forms one of said regions and/or parts which are influenced by the forces and loads, and the skirt is an element that forms a downward tubular body below a ring belt and is usually used for internal combustion engines having a high power stroke. To allow the piston to run free in the cylinder, the piston should, for example, have a small clearance to the running surface of the cylinder. In particular pinless pistons are influenced in its expansion and/or retraction behavior in an outstanding manner due to the central force transmission from the connecting rod into the piston, which leads to the necessity of a special piston design.
In the prior art, a piston is joined by using a transversal pin, which transmits the load from the piston to the connecting rod and assures a relative movement in between. The load transmission happens from the piston to the pin and then from the pin to the connecting rod. Taking into account this arrangement, there is a connecting rod cylindrical surface in contact to a pin cylindrical surface, which produces sliding movements during the working process.
Since high loads rise by the combustion adjacent to the piston crown, this generates mechanical expansion of piston skirt, where transversal force components affect said piston skirt and cause strains to the piston body. In course of time, these loads can cause high wear and increase the friction forces of the piston in operative interconnection with the cylinder.
Document U.S. Pat. No. 3,765,307 A describes a piston for a pinless assembly in which the piston base is designed with a transverse open slot allowing the connecting rod a reciprocating movement in operative interconnection with the crankshaft. The connection between the piston and the connecting rod is provided directly by a concave open surface in the piston centre and a convex cylindrical surface at the small end of the connecting rod. Two open bores are provided diametral in the piston body, being those bores located in the opposite, extending parallel to the longitudinal axis of the piston and the only function of these bores is to reduce the weight of the piston.
Document JP 2000/065205 A describes a piston with a structure to connect the piston with a connecting rod of the piston without making use of a piston pin, maintaining the high rigidity of the piston and facilitating the connection between the piston and the connecting rod. The piston described has an out jutting part of the piston body having an enlarged tip end. The piston is connected to the connecting rod by use of said jutting part, and there is no need to provide a stopper for the insertion of the piston, and whereby the need of a hollow part in the piston is avoided. By means of the jutting part the connecting rod is linked to the piston by means of negative locking dispensing a piston pin. The piston connecting structure leads to a negative deformation behavior upon high loads from the connecting rod into the piston.
Document EP 1 983 232 A1 describes a crankshaft drive for internal combustion engines and compressors having a piston and an arrangement to a connecting rod. The connecting rod is linked to the piston by means of positive locking means. The connecting rod and the piston have integrated positive locking elements engaged to each other, preferably arranged in the main load direction of the connecting rod. The connecting rod represents a swivel on the piston-facing end with an external circumferential curve contour where a joining end shell is located towards the back of the piston.
Document US 2009/0084260 A1 describes a piston for a pinless assembly with cavities in the piston that form mirrored piston halves, these cavities being perpendicular to the annular pocket receiving the small bearing end of the connecting rod and extending from the base of the piston skirt to the piston crown. The small end can be provided with an external bearing material on a convex outer surface, and the small end is unobstructed from lateral movement along the opening over a full stroke of the piston.
A common disadvantage of known piston designs performed for a pinless assembly to the connecting rod in the prior art rises due to a specific deformation behavior in the piston structure upon the combustion load on the top of the piston limiting the combustion chamber and in particular upon the force transmission from the connecting rod into the joint claw and thus into the inner section of the piston. The periodic deformation of the piston structure in the active use of the piston fulfilling a reciprocating stroke movement leads to a detrimental influence on the clearance between the piston skirt and the cylinder running surface.